You have set your video card graphic settings
TOO LOW, or your
video card driver needs to be updated.
Please ensure that you have the latest video card driver installed from
http://www.nvidia.com and leave it at the "default" settings. Try Final Fantasy XI at the default settings FIRST, then raise or lower the video card graphics quality/performance one notch at a time until it is to your satisfaction of speed and quality.
Setting everything to "low" may have involved accidentally turning off a vital function.